1. Field of the Invention
The present invention relates to a polymer battery having a high degree of safety, and to a method for its manufacture.
2. Prior Art
Advances over the past few years in electronics equipment have led to smaller sizes, lighter weights and higher energy densities, and also to a desire in the industry for the development of secondary batteries which can be recharged many times. Lithium secondary cells and lithium ion secondary cells in which the electrolyte is a non-aqueous solution rather than an aqueous solution have attracted particular interest.
In solution-type lithium secondary cells where lithium metal or a lithium alloy serves as the negative electrode, thread-like bodies of lithium crystal known as dendrites form on the negative electrode with repeated charging and discharging, resulting in undesirable effects such as short-circuiting of the electrodes. Hence, a need has been felt for a solid polymer electrolyte that inhibits dendrite deposition and also has the properties of a separator.
Lithium ion secondary cells were developed to resolve the problem of dendrite formation in lithium secondary cells. Yet, because the separator used in lithium ion secondary cells to prevent short-circuiting between the electrodes is incapable of adequately retaining the electrolyte, leakage of the electrolyte solution tends to arise, making it necessary to use a metal can as the outer enclosure. This increases production costs for the battery and prevents a sufficient reduction in battery weight from being achieved. Therefore, to eliminate electrolyte leakage and at the same time reduce the weight of the cell, a need has similarly arisen in lithium ion secondary cells for a very safe polymer electrolyte which also functions as a separator.
Vigorous efforts are thus underway to develop polymer electrolytes prepared with fluoropolymer materials.
Examples include physically crosslinked gels arrived at using such fluoropolymers as polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-chlorotrifluoroethylene (CTFE) copolymers (P(VDF-CTFE)), vinylidene fluoride-hexafluoropropylene fluororubbers, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene fluororubbers and vinylidene fluoride-tetrafluoroethylene-perfluoro(alkyl vinyl ether) fluororubbers.
Such fluoropolymers are known to have good chemical stability to the electrolytes and ions in the solutions used in batteries. For example, U.S. Pat. No. 5,296,318 and U.S. Pat. No. 5,418,091 describe both a gelled electrolyte containing a lithium salt dissolved in a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), abbreviated hereinafter as xe2x80x9cP(VDF-HFP),xe2x80x9d and also a lithium intercalation cell using the gelled electrolyte. These cells have a better ionic conductivity and discharge characteristics, and in particular a better rate capability, than cells made using earlier gelled electrolytes. That is, increasing the discharge current does not lower to any great degree the discharge capacity.
Yet, although gelled electrolytes made with PVDF-based copolymers such as P(VDF-HFP) copolymers have excellent properties, they also have a number of serious drawbacks.
The copolymerization involved in formation of the PVDF copolymer lowers the crystallinity of the polymer, making it subject to swelling by the electrolyte. Hence, in spite of the good electrical properties achieved, PVDF copolymers are more prone to deformation and have a lower physical strength than PVDF homopolymers. This appears to be attributable to the essential nature of the material. As a result, a film thickness of at least 60 xcexcm is required for practical use.
Such a large thickness is clearly a drawback when compared with the normal film thickness of 25 xcexcm in separators currently used in conventional solution-type lithium ion cells. The inability to achieve a lower film thickness in lithium ion secondary cells that use a solid electrolyte has until now made it impossible to exploit the considerable practical advantages of such cells.
Another problem with such PVDF-based copolymers is that, because they are polymerized as copolymers, they have a structure in which crystallization has been inhibited to a great degree, and thus melt at a lower temperature. For example, PVDF homopolymer has a melting point of 170xc2x0 C., whereas P(VDF-HFP) copolymer has a melting point of 140xc2x0 C.
Furthermore, in the gelled state containing a large amount of electrolyte solution, the gel film distortion temperature is lower than the melting point of the polymer by itself. In fact, heat distortion occurs at 130xc2x0 C. in a gel film made with PVDF homopolymer, whereas it occurs at about 90xc2x0 C. in a gel film made with P(VDF-HFP) copolymer.
Because the heat distortion temperature in the gelled state is low, at elevated temperatures, the separator has a lower strength and is softer than at room temperature, making it more likely for short circuits to occur between the positive and negative electrodes. For example, in cases where expanded metal is used as the current collector, the electrodes cut into the expanded metal. Local thinning occurs in corresponding portions of the PVDF-based copolymer electrolyte, increasing the likelihood of shorting between the positive and negative electrodes. This is a major obstacle to battery production.
Also, the use of a fluoropolymer electrolyte in electrochemical devices such as lithium ion secondary cells and electrical double-layer capacitors often leads to problems with adhesion of the electrolyte (separator) to the electrodes and current collectors. Inadequate adhesion can result in poor battery storage properties. Storage of the battery at room temperature or at an elevated temperature (e.g., 40xc2x0 C., 60xc2x0 C., 80xc2x0 C., 100xc2x0 C.) results in a deterioration in the capacity and frequent internal shorting. Moreover, lowering the melting point places limits on use of the battery at high temperatures and, as noted above, compromises the high-temperature storage properties.
Because fluoropolymers have an inherently low surface energy and thus do not adhere well to many substances, sufficient adhesion to the positive and negative electrodes cannot be achieved when a fluoropolymer electrolyte is disposed as an electrolyte film between the electrodes.
Quoting directly from JP-A 11-312535:xe2x80x9cFluoropolymers with a weight-average molecular weight of at least 550,000 exhibit excellent adhesion to the active material layers of positive and negative electrodes. It is therefore possible to bond a solid or gelled polymer electrolyte with an electrode active material layer to a sufficient adhesive strength, thus lowering internal resistance within the electrodes and achieving good charge/discharge cycle properties.xe2x80x9d However, the degree of swelling by the fluoropolymer varies depending on the type of electrolyte solution used, and so sufficient adhesive strength is not achieved with all electrolyte solutions.
The heat distortion temperature of a gel is not readily affected by the molecular weight of the polymer. Hence, adhesion within the high temperature region is inadequate even when a fluoropolymer having a sufficiently large molecular weight is used. For this reason and because fluoropolymers have a large heat expansion coefficient, the electrodes and the electrolyte tend to separate with repeated heat cycling between high temperatures and room temperature.
Polymer batteries must also have a high degree of safety. Electrolytes composed of a lithium-based electrolyte salt such as LiPF6 dissolved in a non-aqueous solvent such as a low-molecular-weight carbonate (e.g., ethylene carbonate, propylene carbonate, diethyl carbonate) have been widely used in prior-art lithium secondary cells because of their relatively high conductivity and stable electric potential.
Yet, in spite of their high performance, lithium secondary cells made with such non-aqueous electrolytes are flammable. For example, if a large current suddenly flows into the cell when a short circuit occurs, the cell heats up, causing the organic solvent-containing electrolyte solution to vaporize or decompose. Gas generated as a result may damage or rupture the cell, or even cause it to ignite. Fires sometimes occur because of internal heating due to excessive charging of the cell, and there is even a danger of fire from short circuits caused by the puncture of a charged cell with a nail or other sharp object.
A polymer electrolyte must therefore also have the ability to prevent the cell from igniting. It is thus essential to increase safety by minimizing evaporation of the liquid electrolyte and creating a state in which the electrolyte solution cannot readily vaporize even if the temperature at the interior of the cell rises significantly, and moreover to select a component, namely a polymer, which inhibits electrolyte vaporization in the electrolyte/polymer mixture referred to throughout this specification as the gel.
However, the above-described fluoropolymers have a low affinity to electrolyte solutions. Forming a complex of such a fluoropolymer with the electrolyte solution and rendering the complex into a gel does not in any way alter the rate of electrolyte evaporation, and thus cannot increase the safety of the cell.
It is therefore an object of the present invention to provide a polymer battery which has a high safety, good heat cycling resistance and robust characteristics even when held at a high temperature, and is thus particularly suitable for use as a lithium secondary cell or a lithium ion secondary cell. Another object of the invention is to provide a method of manufacturing such polymer batteries.
Accordingly, a first aspect of the invention provides a polymer battery which includes a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes that is composed primarily of a fluoropolymer, and which is made by impregnating the cell assembly with an electrolyte composition containing
(A) an ion-conductive salt,
(B) a solvent in which the ion-conductive salt is soluble, and
(C) a compound having at least two reactive double bonds per molecule,
then reacting the component C compound to form a three-dimensional network structure.
In the polymer battery of the above first aspect of the invention, the electrolyte composition containing components A to C preferably has an ionic conductivity, as measured by the AC impedance method, of at least 1xc3x9710xe2x88x924 S/cm.
A second aspect of the invention provides a polymer battery which includes a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes that is composed primarily of a fluoropolymer, and which is made by impregnating the cell assembly with an electrolyte composition containing
(A) an ion-conductive salt,
(B) a solvent in which the ion-conductive salt is soluble,
(C) a compound having at least two reactive double bonds per molecule, and
(D) a hydroxyalkyl polysaccharide derivative,
then forming a semi-interpenetrating polymer network structure in which molecular chains on the component D polymer are interlocked with a three-dimensional polymer network structure obtained by crosslinking the component C compound.
The polymer battery of the above second aspect of the invention preferably has a ratio (C1/C2)xc3x97100 between the ionic conductivity C1 of an electrolyte composition which contains components A to D and in which components C and D together form a semi-interpenetrating polymer network structure, and the ionic conductivity C2 of an electrolyte composition which contains components A, B and C or components A, B and D and does not have a semi-interpenetrating polymer network structure of from 80 to 100%.
A third aspect of the invention provides a polymer battery which includes a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes that is composed primarily of a fluoropolymer, and which is made by impregnating the cell assembly with an electrolyte composition containing
(A) an ion-conductive salt,
(B) a solvent in which the ion-conductive salt is soluble,
(C) a compound having at least two reactive double bonds per molecule, and
(E) a polyvinyl alcohol derivative,
then forming a semi-interpenetrating polymer network structure in which molecular chains on the component E polymer are interlocked with a three-dimensional polymer network structure obtained by crosslinking the component C compound.
The polyvinyl alcohol derivative E is preferably a polymeric compound containing polyvinyl alcohol units and having an average degree of polymerization of at least 20 in which some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups.
The polyvinyl alcohol derivative E is also preferably a polymeric compound containing polyvinyl alcohol units and having an average degree of polymerization of at least 20 in which some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with both oxyalkylene-containing groups and cyano-substituted monovalent hydrocarbon groups.
Also preferably, the polyvinyl alcohol derivative E is a polymeric compound containing polyvinyl alcohol units and having an average degree of polymerization of at least 20 in which some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with cyano-substituted monovalent hydrocarbon groups. The polymeric compound having substituted thereon cyano-substituted monovalent hydrocarbon groups is preferably included in an amount of 0.1 to 8 wt % based on the compound having at least two reactive double bonds per molecule C. Typically the cyano-substituted monovalent hydrocarbon groups are cyanoethyl groups.
The polymer battery of the above third aspect of the invention preferably has a ratio (C1/C2)xc3x97100 between the ionic conductivity C1 of an electrolyte composition which contains components A, B, C and E and in which components C and E together form a semi-interpenetrating polymer network structure, and the ionic conductivity C2 of an electrolyte composition which contains components A, B and C or components A, B and E and does not have a semi-interpenetrating polymer network structure of from 80 to 100%.
A fourth aspect of the invention provides a polymer battery which includes a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes that is composed primarily of a fluoropolymer, and which is made by impregnating the cell assembly with an electrolyte composition containing
(A) an ion-conductive salt,
(B) a solvent in which the ion-conductive salt is soluble,
(C) a compound having at least two reactive double bonds per molecule, and
(F) a polyglycidol derivative,
then forming a semi-interpenetrating polymer network structure in which molecular chains on the component F polymer are interlocked with a three-dimensional polymer network structure obtained by crosslinking the component C compound.
The polymer battery of the above fourth aspect of the invention preferably has a ratio (C1/C2)xc3x97100 between the ionic conductivity C1 of an electrolyte composition which contains components A, B, C and F and in which components C and F together form a semi-interpenetrating polymer network structure, and the ionic conductivity C2 of an electrolyte composition which contains components A, B and C or components A, B and F and does not have a semi-interpenetrating polymer network structure of from 80 to 100%.
In the polymer battery of any one of the above first to fourth aspects of the invention, the compound having at least two reactive double bonds per molecule C preferably has at least two reactive double bonds per molecule and constitutes at least 1 wt % of the overall electrolyte composition.
A fifth aspect of the invention provides a method of manufacturing a polymer battery, which method includes the steps of:
(a) impregnating an electrolyte composition containing
(A) an ion-conductive salt,
(B) a solvent in which the ion-conductive salt is soluble, and
(C) a compound having at least two reactive double bonds per molecule into a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes; then
(b) reacting component C to form a three-dimensional network structure.
A sixth aspect of the invention provides a method of manufacturing a polymer battery, which method includes the steps of:
(a) impregnating an electrolyte composition containing
(A) an ion-conductive salt,
(B) a solvent in which the ion-conductive salt is soluble,
(C) a compound having at least two reactive double bonds per molecule, and
(D) a hydroxyalkyl polysaccharide derivative into a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes; then
(b) forming a semi-interpenetrating polymer network structure in which molecular chains on the component D polymer are interlocked with a three-dimensional polymer network structure obtained by crosslinking the component C compound.
A seventh aspect of the invention provides a method of manufacturing a polymer battery, which method includes the steps of:
(a) impregnating an electrolyte composition containing
(A) an ion-conductive salt,
(B) a solvent in which the ion-conductive salt is soluble,
(C) a compound having at least two reactive double bonds per molecule, and
(E) a polyvinyl alcohol derivative into a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes; then
(b) forming a semi-interpenetrating polymer network structure in which molecular chains on the component E polymer are interlocked with a three-dimensional polymer network structure obtained by crosslinking the component C compound.
An eighth aspect of the invention provides a method of manufacturing a polymer battery, which method includes the steps of:
(a) impregnating an electrolyte composition containing
(A) an ion-conductive salt,
(B) a solvent in which the ion-conductive salt is soluble,
(C) a compound having at least two reactive double bonds per molecule, and
(F) a polyglycidol derivative into a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes; then
(b) forming a semi-interpenetrating polymer network structure in which molecular chains on the component F polymer are interlocked with a three-dimensional polymer network structure obtained by crosslinking the component C compound.
The present invention resolves a number of problems with prior-art polymer batteries in which fluoropolymers are used as an electrolyte material, thereby making it possible to fully and effectively exploit the excellent properties of fluoropolymers.
That is, we have found that a polymer battery which includes a cell assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes that is composed primarily of a fluoropolymer has many desirable and useful properties when manufactured by either:
(i) impregnating the cell assembly with an electrolyte composition containing (A) an ion-conductive salt, (B) a solvent in which the ion-conductive salt is soluble and (C) a compound having at least two reactive double bonds per molecule, then reacting component (C) to form a three-dimensional network structure; or
(ii) impregnating the cell assembly with an electrolyte composition containing (A) an ion-conductive salt, (B) a solvent in which the ion-conductive salt is soluble, (C) a compound having at least two reactive double bonds per molecule, and any one of (D) a hydroxyalkyl polysaccharide derivative, (E) a polyvinyl alcohol derivative or (F) a polyglycidol derivative, then forming a semi-interpenetrating polymer network structure in which molecular chains on the component D, E or F polymer are interlocked with a three-dimensional polymer network structure obtained by crosslinking the compound having at least two reactive double bonds per molecule of component C.
The inventive polymer batteries made in either of these ways have improved adhesion and are thus far less subject to separation of the electrodes from the electrolyte (separator) due to repeated heat cycling between an elevated temperature and room temperature, making it possible to prevent a rise in internal resistance. Moreover, as shown in FIG. 9 described below, the rate of evaporation is so slow compared with that for prior-art fluoropolymer electrolytes that vaporization takes place only with difficulty, making it possible to effectively suppress evaporation of the electrolyte solution. The result is a polymer battery which does not ignite from internal heat, and is thus very safe. In addition, the polymer batteries of the invention have a high heat cycling resistance, and are thus able to sustain an excellent rate capability even when held at a high temperature. This combination of features make the inventive polymer batteries particularly well suited for use as lithium secondary cells and lithium ion secondary cells.